1. Field of the Invention
The present invention relates to oxide superconductors containing rare earth elements having a novel structure and a process for their production.
2. Discussion of Background
Heretofore, superconductors (hereinafter referred to as rare earth superconductors) of the formula REBa.sub.2 Cu.sub.3 O.sub.7-y wherein RE is at least one member selected from the group consisting of Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu and y is the amount of oxygen deficiency, have been known. As a method for producing a bulk material of a rare earth superconductor, there is a method wherein a crystal powder having the above composition is prepared, and then the powder is compacted and sintered. It is also known to prepare it by a solgel method or by a melt process.
The superconductors prepared by such methods are all polycrystals, wherein crystals are randomly oriented, and at the grain boundaries, they contain crystal phases other than REBa.sub.2 Cu.sub.3 O.sub.7-y crystal superconducting phase (hereinafter referred to as a 123 phase), and/or non-crystaline phases, and in many cases they contain air bubbles. A rare earth superconductor has a predetermined direction in the crystals of the 123 phase in which an electric current readily flows. Therefore, the current tends to hardly flow among crystals aligned in different directions. Further, the grain boundary is not a superconductor, and it acts as an insulating layer. Therefore, none of conventional polycrystal rare earth superconductors exhibits a high critical current density.
The degradation in the critical current density attributable to such a grain boundary is known to be a phenomenon observed particularly remarkably in a magnetic field. With respect to the application field of superconductors, they are considered useful primarily as magnets capable of providing strong magnetic fields in the form of wire material or tape material formed in to a coil. Therefore, in order to make a rare earth superconductor to be practically useful, it is believed necessary to prepare a structure wherein crystals of the 123 phase are aligned and the grain boundary is suppressed to obtain a material having a high critical current density even in a strong magnetic field.
As a common method for preparing a ceramic material having such a structure, a method is known wherein a melt is solidified in one direction under a temperature gradient. By this method, it is possible to obtain an aligned ceramic material having a density higher than the one obtainable by a sintering method. However, the 123 phase melts incongruently at a temperature of about 1,000.degree. C. to form RE.sub.2 BaCuO.sub.5 crystals (hereinafter referred to as a 211 phase) and a liquid phase rich in CuO or BaCuO.sub.2. Accordingly, when the melt having the same composition as the 123 phase is cooled, the 211 phase precipitates first, whereby it is impossible to obtain by a usual method to obtain a single crystal or an aligned polycrystal of the 123 phase.
Under these circumstances, a self flux method has been proposed. As shown in FIG. 4, according to this method, when compound AB is subjected to incongruent melting to form a solid phase A and a liquid phase rich in B, the entire composition is shifted towards B (composition Y) so that crystals of AB are grown from the liquid phase rich in B. Accordingly, the solidified product finally obtained will be a mixture of AB and B. Yet, it has a structure wherein B is present at the grain boundary of crystal grains of AB, and AB will not constitute a continuous phase.
In the case of a rare earth superconductor, the 123 phase corresponds to AB in FIG. 4, and likewise, the 211 phase corresponds to A and the liquid phase rich in CuO or BaCuO.sub.2 corresponds to the liquid phase rich in B. It has been reported that in a case where RE is yttrium, the composition with an atomic ratio of Y:Ba:Cu=1:2:3 is shifted to a direction rich in Cu, or rich in Cu and Ba, so that a liquid phase will be formed at a temperature lower than the incongruent melting temperature of YBa.sub.2 Cu.sub.3 O.sub.7-y, and then YBa.sub.2 Cu.sub.3 O.sub.7-y crystals are grown from the liquid phase of Y-Ba-Cu-O system (e.g. Japanese Journal of Applied Physics, Vol. 26, L1425, (1987)).
According to this method, a thin plate single crystal of about 1.times.1 mm or a block polycrystal having a long axis of about 3 mm containing impurities, is obtainable. However, it is difficult to obtain a larger crystal. Besides, the structure tends to be such that the YBa.sub.2 Cu.sub.3 O.sub.7-y crystal grains are surrounded by CuO or BaCuO.sub.2 as an insulator, and it is difficult to obtain a superconductive structure wherein crystals are continuous. In the case of YBa.sub.2 Cu.sub.3 O.sub.7-y, it is particularly difficult to obtain a uniform product having a practical size, since the primary crystal region is narrow, and the melt is likely to undergo phase separation.
In addition to the self flux method, a MTG (Melt-textured growth) method is also reported wherein YBa.sub.2 Cu.sub.3 O.sub.7-y crystals are heated and melted to obtain a mixture wherein a solid phase of Y.sub.2 BaCuO.sub.5 and a liquid phase of Y-Ba-Cu-O system coexist, which is then solidified under a temperature gradient to let YBa.sub.2 Cu.sub.3 O.sub.7-y crystals grow by a peritectic reaction represented by the formula: EQU Y.sub.2 BaCuO.sub.5 +liquid phase.fwdarw.YBa.sub.2 Cu.sub.3 O.sub.7-y
(Physical Review B, Vol. 37, 7850, (1988)).
However, the product was the one which contains in addition to YBa.sub.2 Cu.sub.3 O.sub.7-y crystals, Y.sub.2 BaCuO.sub.5 crystals and other grain boundary phases (CuO, BaCuO.sub.2, non-crystalline phase). Because the liquid phase of Ba-Cu-O system solidifies at lower temperature than 123 phase, it spreads as an insulating layer along the grain boundaries of the YBa.sub.2 Cu.sub.3 O.sub.7-y crystals. Such an insulating layer adversely affects the electrical conductivity characteristics. Further, although the YBa.sub.2 Cu.sub.3 O.sub.7-y phase has certain orientation, the crystals are in contact with each other with certain angles.
A QMG (Quench and Melt Growth) method (1988 Autumn 49th Lecture Meeting of the Japan Society of Applied Physics, 4a-pavilion B-2) is a method wherein a sample melted and then quenched, is again partially melted, followed by solidification. By the first melting and quenching, a structure is formed wherein Y.sub.2 O.sub.3 is dispersed in the form of fine particles, and it is again melted and solidified to let the following two step peritectic reaction take place: EQU Y.sub.2 O.sub.3 +liquid phase.fwdarw.Y.sub.2 BaCuO.sub.5 EQU Y.sub.2 BaCuO.sub.5 +liquid phase.fwdarw.YBa.sub.2 Cu.sub.3 O.sub.7-y
In this reaction, Y.sub.2 O.sub.3 first dispersed is fine particles, and the finally obtained structure will be such that Y.sub.2 BaCuO.sub.5 is finally dispersed in the YBa.sub.2 Cu.sub.3 O.sub.7-y matrix, whereby the uniformity of the entire solidified product will be improved. However, a quenching operation is required. Accordingly, there is a limitation in the shape of the product thereby obtained.